This invention relates in general to the field of DC-DC converters. In particular, this invention relates to boost mode DC-DC converters operating within portable receivers.
Portable receivers such as pagers operate from a single cell battery. The pagers may be controlled by a microcomputer which requires more operating voltage than is available from the single cell battery. Consequently, a DC-DC converter is used to boost the battery voltage to a converted voltage high enough to operate the microcomputer. In place of, or in addition to the microcomputer, the boosted voltage may supply power to other circuits.
In portable battery operated products, such as pagers, it is desirable to have the lowest possible current drain in order to maximize battery life. Since the bias circuits of DC-DC converters consume current, it is desirable to switch the bias circuits off when power conversion is not required. In the past, DC-DC converters have been switched off with a predetermined duty cycle and a predetermined period. Such a method is shown in U.S. Pat. No. 4,634,956 to Davis et. al.
The DC-DC converter described in said patent, periodically switched an inductor to ground using an NPN switching transistor, and in releasing the inductor, developed a voltage potential across the inductor. When the voltage exceeded a predetermined voltage, a rectifying diode transferred the energy in the inductor to a storage capacitor to develop a converted voltage potential. Using a closed loop feedback system, the duty cycle of the periodic switching to ground of the inductor was adjusted to maintain a regulated voltage. However, when the converted voltage was substantially below the regulated voltage, a predetermined maximum duty cycle was selected for operation under worst case conditions. As a result of the predetermined maximum duty cycle, the current through the inductor builds to a large value This large value caused the inductor to saturate, the NPN switching transistor to be driven out of saturation, and excessive current to be delivered through the rectifying diode. This reduced the conversion efficiency of the DC-DC converter.
When the DC-DC converter was switched off for a predetermined time, circuits were powered by the energy stored within a capacitor coupled to the converted voltage. The capacitor was allowed to discharge during the predetermined time when the DC-DC converter was switched off. Then when the DC-DC converter was switched back on for a second predetermined time, the capacitor was recharged. During the recharge process, the DC-DC converter operated at the aforementioned maximum duty cycle, thereby decreasing the efficiency of the conversion process.
Additionally, the DC-DC converter was powered off and on with a fixed period and for a fixed "on" duration with a signal generated by a microcomputer based pager decoder. The period and "on" duration were selected to insure that the converted voltage does not decay excessively during the off interval under worst case load and circuit parameter conditions. As a result, under typical conditions the "on" duration was excessive which further degraded efficiency. Additionally, the microcomputer had to remain in a relatively high power operating state in order to power off and on at the fixed period.
Thus, although said patent described a method for the substantial reduction of current within a paging receiver when operating in a reduced power mode, the conversion efficiency of the DC-DC converter degraded, and the microcomputer operated in a relatively high power operating state in order to provide for the substantial current reduction.
Additionally, the DC-DC converter of the aforementioned patent had three operating modes, the first being a low current mode wherein the DC-DC converter powers off and on, which was typically used while the pager operated in a battery saving mode. The second being a medium current load, which was typically used while receiving and processing data. And the third being a high current load, which was typically used while the pager was reading the code plug. The second and third modes are described in U.S. Pat. No. 4,355,277 to Davis et al. The microcomputer issued commands which operated the DC-DC converter in each of these three modes, which was an additional burden for the microcomputer.